This invention relates to hydraulic devices of the geroter type and more particularly to a pressure-balancing arrangement for geroter type devices employing rollers as teeth of one of the displacement members therein.
Hydraulic devices of the type commonly referred to as geroters comprise an internally-toothed ring member eccentrically receiving an externally-toothed star member. The star member usually has one less tooth than the ring member to define a number of volume chambers which expand and contract upon one member's hypocycloidal movement about the axis of the other. To maintain volumetric efficiency within such devices, leakage between volume chambers at high pressure and those at low pressure must be minimized. Thus the fit between mating teeth is critical and the teeth of one member must be precisely formed to provide an accurate fit with the teeth of the other member. When the teeth wear, clearance between the members becomes excessive and leakage with resultant inefficiency occurs.
The prior art has attempted to overcome the problems associated with the close fit between the ring member and star member by forming the teeth of the ring member from cylindrical rollers rotatably positioned in pockets formed in the ring member. Geroter gear sets utilizing rollers as the teeth of the ring member may be classified, or distinguished, by the relationship, for each particular geroter set, between the roller diameter and pocket diameter, as well as by the relationship between the "tangent circle" of the ring-roller assembly (i.e., a circle tangent to the inner peripheries of the rollers centered within their respective pockets) and the "average diameter" of the star member (i.e., the average of the major and minor diameters of the star member).
Thus, a geroter set is referred to as having an "interference fit". When each of the rollers has substantially a bearing fit with its respective pocket (i.e., a diametral clearance between the roller and pocket of less than about .0015 inches (.038 mm) ), and the average diameter of the star member "interferes" with the tangent circle of the ring roller assembly (i.e., the average diameter is larger than the tangent circle by as much as about .0010 inches (.025 mm), or more). In such a geroter set, the interference fit precludes any biasing or inward radial movement of the rollers.
A geroter set is referred to as having a "non-interference fit" when each roller has a bearing fit with its respective pocket, as in the case of the interference fit, but there is a clearance between the average diameter of the star member and the tangent circle of the ring-roller assembly (i.e., the average diameter is dimensionally larger than the tangent circle by about .0010 inches (.025 mm), or more). In a geroter set having a non-interference fit, radial movement or biasing of the rollers is possible because of the clearance between the average diameter and the tangent circle.
A geroter set is referred to as having a "loose fit" when each roller and its respective pocket have more diametral clearance than the running clearance attributable to a bearing fit relationship. Typically, with a loose fit geroter set there is a diametral clearance between each roller and its respective pocket of about .003 inches (.076 mm) to about .005 inches (.127mm). With the rollers loosely received in the pockets, the relationship of the average diameter and the tangent circle is less critical and there may be a nominal interference between the average diameter and the tangent circle, although, typically, a clearance is provided.
Therefore, because the present invention relates to biasing of the rollers, it will be understood by those skilled in the art that the invention would be advantageous only when utilized with a geroter set having a non-interference fit or with a geroter set having a loose fit, and, preferably, with a geroter set having a non-interference fit to minimize leakage problems.
Among the prior art methods of biasing the rollers inward into the sealing contact with the star member, one known balancing arrangement taps high pressure from a source, such as the valving or one of the volume chambers, and directs it constantly to the pocket behind the roller. With such an arrangement, volumetric efficiency is seriously reduced because of constant leakage which occurs when the rollers remain continuously biased by fluid at high pressure.
It is thus a principal object of the subject invention to provide a hydrostatic pressure biasing arrangement for rollers used as chamber-forming elements in a geroter type device, which arrangement is operable to control the magnitude and direction of the force applied to the rollers to fully balance the geroter arrangement by use of pressure from adjacent chambers while also providing means to support the rollers in their proper geometric arrangement with respect to the teeth of the star member.
In accordance with the invention, there is provided a geroter comprising a roller-type, internally-toothed ring member eccentrically receiving an externally-toothed star member having fewer teeth to define a plurality of volume chambers. Each roller in the ring member is rotatably disposed within a pocket of diameter slightly greater than that of the roller. Each pocket has a plurality of recesses formed therein which are separated from one another by a roller support surface or land and communicated with one another by grooves or depressed areas extending around or through the support surfaces.
The recesses are particularly configured and positioned to develop a controlled, essentially hydrostatic, pressure pattern biasing the roller toward the corresponding star tooth when the roller is positioned between chambers at high and low pressures. The pressure pattern is characterized by being symmetrically distributed over a portion of the pocket to produce a controlled, resultatnt force directed radially-inwardly and of sufficient magnitude to adequately seal high pressure chambers from low pressure chambers.
Additionally, the roller support surfaces maintain the rollers in their proper geometric relationship even though the recesses provide an "excessive clearance" necessary to establish a sufficient resultant force.
It is thus an object of the subject invention to provide high volumetric efficiency in a fluid-operated device of the geroter type even though tooth contact wear has occurred.
Yet another object of the invention is to provide pressure-balancing means in a fluid-operated device of the geroter type acting in a substantially-radial direction and exerting a predetermined resultant force between the teeth of the ring and star members to effectively seal high pressure chambers from low pressure chambers therein.
Still other objects of the invention reside in a radial, pressure-balancing arrangement employed in fluid-operated devices of the geroter type which provides a high starting torque and a compensation for pressure deflection.